Chapter 3 Loaders and LinkersPurpose and FunctionKinds of LoadersKinds of loaders (cont.)Slide 5Methods of RelocationMask Method of RelocationProgram LinkingLOADINGData Structures for LoadingChapter 3Loaders and LinkersPurpose and FunctionPlaces object program in memoryLinking–Combines 2 or more obj programsRelocation–Allows loading at different locationsLinkage Editor–Provides linking without loadingKinds of LoadersAbsolute–Single pass–Checks for correct header record–Checks for sufficient available memory–Moves each text record to proper location–Upon seeing END passes control to the pgmKinds of loaders (cont.)bootstrap–A special absolute loader–ROM–Loads the OSKinds of loaders (cont.)Relocating–Modifies appropriate addresses–Loads object program at a variety of locations–May perform loading during execution (repeatedly)–Allows for multiple programs (multiprocessing)–System libraries require relocationMethods of RelocationModification recordsUse absolute addressing and fixed format–No modification records required–Use same text records with flag (relocation bit)–Relocation bits gathered into a mask–If relocation bit is 1, add starting address to wordMask Method of RelocationHCOPY 000000001077A ^ ^ ^T0000001EFFC1400334810390000362800303000154810613C000300002A0C003900002D ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^FFC 111111111100 all 10 words need modificationT 00001E 15 E00 0C0036 481061 080033 4C0000 454F46 000003 000000^ ^ ^ ^ ^ ^ ^ ^ ^ ^E00 111000000000 instructions 0,1,2 need load addresses T0000391EFFC0400300000030E0105D30103FD8105D2800303010575480392C105E38103F ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^T0010570A8001000364C0000F1001000^ ^ ^ ^ ^ ^ ^The F1 fouls up alignment, thus a new text record has to be started. FIGURE 3.7Program LinkingNecessary for separate CSECTSExternal ReferencesExternal DefinitionsLOADINGForward references to external symbols commonUse 2 pass–Pass 1 assigns addr. to external symbolsProvides a load map (info in symbol table)–Pass 2 performs actual loading, relocation, and linkingData Structures for LoadingESTAB external symbol table – Stores–Names–Addresses–CSECT of external symbolsPROGADDR – program load address–Provided by the OSCSADDR – CSECT addr. of control sect.